Editing IBM Color Graphics Adapter

{{Infobox computer peripheral
| name          = IBM Color Graphics Adapter
| image         = [[File:IBM CGA card.jpg|260px]]
| caption       = An IBM Color Graphics Adapter (CGA) card
| manufacturer  = IBM
| type          = 8-bit ISA video display adapter
| release_date  = 1981 (with the original IBM PC)
| interface     = 8-bit ISA
| compatible    = [[IBM PC (5150)]], [[IBM PC XT (5160)]], [[IBM PC AT (5170)]], PC-compatible clones
| connectivity  = DB-9 TTL RGBI (drives the [[IBM 5153]]); composite NTSC RCA; light-pen header
}}

The '''IBM Color Graphics Adapter''' ('''CGA''') is the original 8-bit ISA colour graphics card IBM released alongside the [[IBM PC (5150)]] in 1981. It uses a Motorola MC6845 CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the [[IBM 5153]]) or a composite TV through an NTSC encoder.

== Video Modes ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''CGA video modes'''
! Mode !! Resolution !! Colours !! Notes
|-
| 0 || 40 × 25 text || 16 fg / 8 bg || Colour burst on
|-
| 1 || 40 × 25 text || 16 fg / 8 bg || Colour burst suppressed
|-
| 2 || 80 × 25 text || 16 fg / 8 bg ||
|-
| 3 || 80 × 25 text || 16 fg / 8 bg || Colour burst suppressed
|-
| 4 || 320 × 200 graphics || 4 from fixed palettes || Palette 0: cyan/magenta/white; Palette 1: green/red/yellow
|-
| 5 || 320 × 200 graphics || 4 (greyscale on composite) || Composite "alternate" palette
|-
| 6 || 640 × 200 graphics || 2 (background + foreground) || Highest standard resolution
|}

Several undocumented modes can be coaxed out of the 6845 (160 × 100 16-colour text-block mode, composite NTSC artefact colour, and various demoscene techniques), but they are not part of the BIOS interface.

== Hardware ==

* '''MC6845 CRTC''' — clock and sync generation, addressing.
* '''16 KB video RAM.'''
* '''TTL RGBI output''' on DB-9 (drives the [[IBM 5153]]).
* '''Composite NTSC output''' on RCA jack.
* '''Light-pen connector.'''

== Common Faults ==

* '''Snow''' — the CGA "snow" effect is not a fault but a design quirk: the video RAM is single-ported and CPU writes during the active raster cause briefly-visible noise. Most CGA-aware games wait for the horizontal retrace to avoid it.
* '''No video, beep code 1 long + 2 short''' — reseat the CGA card first.
* '''Missing one colour channel''' — bad output buffer at the DB-9, or cracked solder joint on the connector.
* '''Smoke from a discoloured tantalum''' — replace the failed tantalum and any series resistor that may have died with it.

== References ==

* [https://en.wikipedia.org/wiki/Color_Graphics_Adapter Color Graphics Adapter — Wikipedia].

== Related Pages ==

* [[IBM PC (5150)]]
* [[IBM 5153]]
* [[IBM Monochrome Display Adapter]]
* [[IBM PC (5150) Capacitor Replacement Guide]]

[[Category:IBM]]